1. Field of the Invention
The present invention pertains to a method for providing a stable plasma useful in the etching of films. The method is particularly useful in the etching of silicon-containing films which differ in overall chemical composition, when it is desired to simultaneously etch such films at a substantially equivalent etch rate. For example, adjacent areas of a semiconductor substrate where polysilicon or silicon is doped and undoped can be etched at substantially the same rate. Further, the method provides critical dimension bias control for the feature being etched. The stable plasma provided by the method makes possible a reliable timed etch end point. And, finally, the method offers the advantage that the process chamber in which the etching is carried out remains particularly clean during the etch process, with minimal etch byproduct deposited upon chamber surfaces.
2. Brief Description of the Background Art
K Suzuki et al., in an article entitled xe2x80x9cPower transfer efficiency and mode jump in an inductive RF dischargexe2x80x9d, Plasma Sources Sci. Technol. 7 (1998) 13-20, describe a plasma density jump observed in a large-diameter (50 cm) high density ( greater than 1011 cmxe2x88x923) plasma, where the plasma is produced in a few mTorr argon by inductive RF discharge using a conventional external antenna or a plasma-immersed internal antenna. A mechanism for the density jump is explained in terms of the density dependence of the power transfer efficiency. The density jump is said to correspond to the mode jump from the capacitive to the inductive discharges. Representative formulae are given for capacitive power absorption, Pcap and inductive power absorption Pia.
In recent process development work related to a method for etching silicon-containing layers, a method for obtaining simultaneous cleaning of etch byproducts from the etch chamber wall was discovered. This technology is disclosed in pending U.S. patent application, Ser. No. 08/969,122 of Qian et al., Entitled: xe2x80x9cSelf-Cleaning Etch Processxe2x80x9d, filed Nov. 12, 1997, and is hereby incorporated herein by reference. Subsequently, in a related development, a method was developed which permitted simultaneous etching of silicon-containing layers having different dopant concentrations where the etch rate was substantially uniform across the differing materials. This technology is disclosed in pending U.S. patent application, Ser. No. 09/116,621 of Nallan et al., Entitled: xe2x80x9cProcess For Etching Silicon-Containing Layers On Semiconductor Substratesxe2x80x9d, filed Jul. 15, 1998, and is hereby incorporated herein by reference. This second patent application is a continuation-in-part of the Qian et al. application described above, and both of these applications are assigned to the assignee of the present invention.
These patent applications describe high density plasma etching in a dual power processing apparatus. In particular, in the dual power processing apparatus, the shape of the processing chamber may be specially designed and the power applied to the apparatus for generation of the plasma is separately controlled from the power applied to bias the substrate. This has a substantial effect on the etch results obtained, as is evident in the disclosure of the inventions of Qian et al. and Nallan et al. referenced above.
The Qian et al. and Nallan et al. applications disclose the use of a plasma source gas which includes four main components: a bromine-containing gas including one or more of HBr, Br2, and CH3Br; a chlorine-containing gas including one or more of Cl2, and HCl; an inorganic fluorinated gas including one or more of NF3, CF4, and SF6; and an oxygen-comprising gas. Typically, the oxygen-comprising gas is a mixture of an inert gas with oxygen, such as Hexe2x80x94O2, where the O2 ranges from about 20% to about 30% by volume of the gas mixture. The volumetric flow ratio of the various plasma source gas components is selected so that the relative etch rates of the different silicon-containing films vary by less than about 10%.
In one embodiment of the disclosed technology a two step etching process is used. In the first step, the plasma source gas includes all of the four components described above. The CF4 component both tends to equalize the etch rate among the different silicon-containing films (such as doped and undoped silicon) and tends to assist in removal of etch byproducts from the etch process chamber wall. However, with CF4 present, the selectivity ratio of polysilicon:silicon oxide is lower. This is important, for example, when a silicon film (doped in some areas) overlies a thin silicon-oxide film (functioning as a gate for a transistor). The etch rate for the silicon oxide in the four-component plasma etchant system is sufficiently rapid relative to the polysilicon etch rate that there is a danger of etching through the thin silicon oxide film before the polysilicon etching process can be terminated. To solve this problem, in a second step (which may be part of a continuous process), the CF4 is removed from the plasma source gas so that a xe2x80x9csoft landingxe2x80x9d is achieved when the end point of the polysilicon etch process occurs. By removing the CF4 and increasing the selectivity (etch rate ratio) of polysilicon to silicon oxide, it is possible to etch up to a thin underlying silicon oxide layer without the danger of etching through.
It was desired to use a timed etch end point for the first etch step described above and a process variable monitored etch end point for the second step. However, there was difficulty in using a timed etch end point for the first step, because the etch rate was not sufficiently predictable.
Subsequent to development of the technology described above, we discovered that under some process conditions an unstable plasma developed. A brightening of the plasma was observed, with an accompanying change in etch rate. This unstable plasma was observed in some instances and not in others. We investigated this unstable plasma in an attempt to obtain a more predictable etch rate, with the goal in mind of enabling the use of a timed etch end point for the first etch step.
In accordance with the present invention, the etch plasma density is controlled to provide plasma stability. We have discovered that it is possible to operate a stable plasma with a portion of the power deposited to the plasma being a capacitive contribution and a portion of the power deposited being an inductive contribution. In particular, a stable plasma may be obtained within two process regions. In the first region, the gradient of the capacitive power to the power applied to the inductively coupled source for plasma generation [∂Pcap/∂PRF] is greater than 0. In the second region, plasma stability is controlled so that [∂Pcap/∂PRF] is less than 0 and so that Pcap less than  less than PRF. Typically, the magnitude of Pcap is about 10% or less of the magnitude of PRF. In these equations, Pcap is the calculated capacitive power deposited to the plasma, and PRF is the actual power applied. The stability of power deposition to the substrate (workpiece) is achieved by choosing both the RF power level applied to the inductively coupled plasma generation source and the operating pressure in the process chamber to provide a stable plasma. It is preferred to operate under conditions where [∂Pcap/∂PRF ] is greater than 0, because the etch results are superior, due to a more favored ion to neutral ratio in the plasma.
We have also discovered that, at a given application of power to the plasma generation source, the stability of the plasma may be extended by increasing the pressure in the etch process chamber. This enables operation of the etch process using lower power application for plasma generation. The stable plasma operating regime is overlaid upon the process which permits etching of different silicon-containing layers at substantially the same etch rate, while reducing the need to clean the process chamber. Further, process conditions may be adjusted to provide the desired etch profile, with reduced microloading effect; the desired selectivity toward the silicon-containing layers relative to an adjacent layer of material (such as a patterned resist layer); and etch uniformity across a substrate. The stable high density plasma combined with the etch chemistry and multiple step method disclosed herein provides a more uniform three dimensional etch across the wafer, in general.
In particular, the plasma stability may be controlled by selection of operating conditions that provide stable plasma deposition of the RF power supplied to generate the plasma. The process conditions are provided by choosing both the RF power applied to the inductively coupled source for plasma generation and the etch chamber operating pressure, in view of the particular gases from which the plasma is generated.
Another potential method for determining whether the plasma will be stable is to actually measure the capacitive and inductive components of the plasma. In this case the measured capacitive component is Pcap2 and the measured inductive component is Pind. In this method, the magnitude of Pcap2 is typically less than or equal to 10% of the magnitude of Pind in the stable plasma region. An approximate Pcap may be determined by monitoring the amount of RF current flowing to the chamber ground.
When the apparatus used is one for which no data is available, it is possible to determine the two process regions in which a stable plasma may be obtained using minimal experimentation. The first stability region, in which [∂Pcao/∂PRF] is greater than 0 may be determined by setting Pcap2 or Pcap and PRF so that this condition is met at a relatively low RF power and at a relatively low process chamber pressure (for example, and not by way of limitation, about 2 mTorr). The RF power is then increased until an instability in the plasma is observed. This procedure is repeated for a series of increasing process chamber pressures. The maximum chamber pressure to be used may be determined by the characteristics of the etched feature, etch uniformity across the wafer, selectivity between masking layers and the silicon-containing layer to be etched, and other parameters related to the resultant etched product.
To determine the second stability region, in which the [∂Pcap/∂PRF] is less than 0 and Pcap less than  less than PRF: PRF may be set at a value about 100% greater than the maximum RF power determined to produce a stable plasma in the first stability region (and at the maximum process chamber pressure used which provided an acceptable result). The process chamber pressure is then decreased until an unstable plasma is observed. The RF power is then decreased and the process chamber pressure at which instability occurs is observed for a number of decreasing RF power applications.
A stable plasma enables use of a timed etch end point for etching a particular film or layer. Although the method of the invention is applicable to a variety of materials, the method is described in detail herein with reference to a semiconductor structure including a silicon-containing layer.
The process conditions for a stable plasma must be overlaid on the effect of the process variables on etch characteristics (particularly of materials such as masking layers, barrier materials and dielectric gate materials) to provide an acceptable manufacturing etch process. Etch characteristics such as, but not limited to, etch rate, selectivity toward various substrate materials, profile control of the etched feature, and etch uniformity across the substrate must be satisfactory at the process conditions which provide the stable plasma. Further, the etch process must be sufficiently clean so that an acceptable number of substrates can be processed prior to the need to wet clean the process chamber.
In an embodiment of the invention where a silicon-containing layer is being etched, the process gases from which the plasma is produced include one or more of HBr, Br2, and CH3Br; a chlorine-containing gas, such as Cl2 and/or HCl; an inorganic fluorinated gas such as NF3, CF4, SF6, and combinations thereof; and an oxygen-comprising gas which is typically diluted with an inert gas. The combination of process gas components is selected to provide the desired selectivity so that the silicon-containing layer will etch at a significantly more rapid rate than an adjacent layer, for example, the masking layer used to pattern the silicon-containing layer. Typically the masking layer is a DUV resist or an I-line resist (which do not contain silicon). The silicon-containing layer may contain doped and undoped regions, and when this is the case, the combination of process gas components is further selected to etch the doped and undoped regions at etching rates which typically do not differ more than about 10%.
The features to be etched into the silicon-containing layers typically have a feature size of less than about 0.5 xcexcm. In addition, frequently the aspect ratio of the features is greater than 2:1. To etch features of this size and have a satisfactory etched structure, there are several critical results which must be achieved during the etch process. The etch rate of the silicon-containing layer should be at about 1,500 xc3x85/min. or higher to be economically advantageous. (When a two step etch process is used, wherein a first portion of the layer is etched rapidly and a second portion approaching the end point is etched more slowly, it is the first portion etch which should be at this etch rate.) The etch selectivity of the silicon-containing layer relative to an adjacent layer (the ratio of the etch rate of the silicon-containing layer to the etch masking layer, for example) should be at least 2:1. When the feature being etched is lines and spaces, the profile angle for the etched feature should range from about 88xc2x0 to about 90xc2x0. The profile microloading should be less than about 2xc2x0, and the max-min etch uniformity across the substrate should not vary by more than about one standard deviation, "sgr", (more than about xc2x12%).
The critical results which must be achieved may vary depending on the feature being etched, but etch rate, selectivity, etch uniformity, and microloading are of constant importance. Whatever the critical results are, these must be overlaid on the requirements for a stable plasma in determining an acceptable operating range within the process variables. In any case, meeting these critical results is extremely difficult when operating within process conditions which produce an unstable plasma.